1. Field of the Invention
The present invention relates to the MoSi2 coating with excellent oxidation resistance and corrosion resistance.
2. Description of the Background Art
Refractory metals such as Mo, Nb, Ta or W have high strength and hardness at a high temperature, and shows more excellent mechanical and thermal properties at a high temperature than other metals because they have low vapor pressures and thermal expansion coefficients. Therefore, the refractory metals have been used as a core material in the field of aerospace, atomic energy, etc.
Among the refractory metals, Mo and W easily react with oxygen even at low temperature, to form volatile MoO3 and WoO3. Accordingly, the use of Mo and W are restricted to non-oxidizing atmosphere. In addition, Nb and Ta react with oxygen at high temperature, to form Nb2O5 and Ta2O5. Since diffusion of oxygen through these oxides is very fast, Nb2O5 and Ta2O5 cannot be used as an oxide film for protecting Nb and Ta.
To improve high-temperature oxidation resistance of the refractory metals or their alloys, metal silicide (MeSi2, Me=Mo, Nb, Ta or W) coating with excellent high-temperature oxidation resistance has been developed. Among the metal silicides, NbSi2 and TaSi2 react with oxygen at high temperature, to form mixed oxide layers including Nb2O5 and SiO2, and Ta2O5 and SiO2, respectively. However, since diffusion of oxygen through Nb2O5 and Ta2O5 layers is very fast, the mixed oxide layers are rapidly grown. When the cyclic oxidation is repeated between high temperature and low temperature, the mixed oxide layer is easily peeled off the substrate by thermal stress due to difference in thermal expansion coefficient between the mixed oxide layer and the substrate. Thus, NbSi2 and TaSi2 are not suitable for a high-temperature oxidation resistance coating to protect the refractory metals or their alloys.
Among the metal silicides, MoSi2 and WSi2 react with oxygen at high temperature, to form MoO3 and SiO2, and WO3 and SiO2, respectively. MoO3 and WO3 are easily vaporized over temperature of about 700 to 800° C., and as a result an adherent and continuous silicon oxide (SiO2) film is formed on the surfaces of MoSi2 and WSi2. Since diffusion of oxygen through the SiO2 film is very slow, the SiO2 film can protect the substrates from further oxidation. However, major obstacle for application of MoSi2 coating is structural disintegration during low temperature oxidation at about 400 to 600° C. (the so-called, pest oxidation), and WSi2 is disintegrated at about 1000 to 1200° C. MoSi2 is known to have more excellent oxidation resistance at a high temperature than WSi2. Accordingly, if MoSi2 is coated on the surface of the refractory metals, the coated refractory metals can be more widely used.
As coating processes to form MoSi2 coating, a slurry method is an easy process to manufacture the MoSi2 coating on alloy, but has a drawback of generating a lot of pores in the coating layer.
Direct coating of MoSi2 by low pressure plasma spraying method is also easy to manufacture the MoSi2 coating on alloy, but cannot properly control compositions and obtain the MoSi2 coating layer without defects.
Reactive diffusion methods such as a pack siliconizing, a chemical deposition and solution growth into molten Si—In alloy is a relatively inexpensive, highly versatile, easily handled, and commercially feasible diffusion coating process. In the pack siliconizing and the chemical vapor deposition, Si is deposited from a gas phase on a surface of a substrate, whereas in the solution growth Si is deposited from liquid phase.
In relation to thermal and mechanical properties, the following three factors mostly influence a commercialization of the MoSi2 coating layer.
(1) Interdiffusion of substrate and MoSi2 coating layer;
(2) Thermal stress due to the difference in thermal expansion coefficients between the substrate and the MoSi2 coating layer (8.5×10−6/° C.) or between the MoSi2 coating layer and the silicon oxide layer (0.5×10−6/° C.); and
(3) Low-temperature oxidation (pest oxidation) of MoSi2 coating layer under air at about 400 to 600° C. and resultant decomposition into MoO3 and SiO2 powders
Accordingly, in the practical applications, the lifetime of the MoSi2 coating layer on the refractory metals or their alloys depends on the using conditions.
In the case of isothermal oxidation (an oxidation for a long time at high temperature), Si is supplied to the substrate due to interdiffusion of the refractory metal substrate and the MoSi2 coating layer. As a result, the MoSi2 coating layer is transformed into an (Me,Mo)5Si3 coating layer without oxidation resistance, and thus cannot be used as a coating layer for protecting substrates. In this case, the increase in the thickness of the MoSi2 coating layer can improve the lifetime of the coating layer.
However, for applications of MoSi2 coating in air at elevated temperatures, service requirements often include conditions of thermal cycling. On reheating, the cracks are closed by thermal expansion and may heal by diffusion. They may reopen or new cracks may form during cooling. During the heating cycle, oxidants can have access to the interior of the coating along these cracks, and oxides may form in the cracks which will prevent complete healing at high temperatures. With repeated temperature cycling, many of the cracks remain open and allow oxidation to continue within the coating. As a result, the substrate is directly exposed to oxygen in the atmosphere, and thus rapidly oxidized.
In addition, when the MoSi2 coating is used at 400 to 600° C., the MoSi2 coating layer is rapidly oxidized to form MoOx and silicon oxides (pest oxidation). During the pest oxidation, the MoSi2 coating layer is decomposed into powders due to volume expansion of about 250% induced by complete oxidation of MoSi2 phase into the MoO3 and SiO2 phases. The decomposed MoSi2 coating layer does not have low-temperature oxidation resistance. Accordingly, the repeated thermal cyclic oxidation resistance, low-temperature oxidation resistance, and high-temperature oxidation resistance should be excellent to wide applications of the refractory metals or their alloys coated with the MoSi2.